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Some pore over men's magazines, with their promises of "washboard abs", for guidance. The more scientifically minded look for experimental data. The latest evidence comes from a group of researchers led by Brian Mautz, then of Australian National University.

They gathered 105 heterosexual Australian women and showed them a series of digitally generated pictures of men in which three bodily characteristics were varied — height, shoulder-to-waist ratio and flaccid penis size. The women were asked to rate the men as sexual partners.

In an article just published in the Proceedings of the National Academy of Sciences, Mautz and his team describe their results. Happily for the insecure, although the women did indeed find a larger penis alluring, it was not the most important factor. (Indeed, above about 3 inches, additional enlargement in genitalia didn't make much difference in attractiveness.) That honor went to the combination of broad shoulders and a narrow waist, which accounted for around three-quarters of the variation in attractiveness all by itself.

Height was also only a weak predictor of appeal. That is odd, says Mautz, because other studies have linked height with all sorts of benefits, from higher salaries to longer lives. The bigger-is-better effect was also subject to diminishing returns: each extra centimeter, whether of height, shoulder width or penis size, was less significant than the previous one. (For those of you measuring at home, remember that 2.54 centimeters equal an inch.)

Nonetheless, even the tallest, broadest-shouldered and best-endowed digital hunks that the researchers generated fell short of perfection. "The optimum values appear to lie outside the tested range," they note, adding that the "maxima are more than two standard deviations from the population mean for each trait."

That means that, for each trait, fewer than 2.5 percent of the men whom women encounter in the real world will be as generously proportioned as they might hope. Men with perfect scores in all three traits will be rarer than hen's teeth.

The study is not perfect. There was no danger of the women mistaking the digital men for the real thing. Other factors — such as social status, for instance — may, in the real world, override the physical preferences that the researchers were measuring.

And it is hard, when all the subjects come from a single country, to disentangle the effects of nurture from those of nature. It is commonly pointed out, for instance, that men's apparent preference for slim women seems to be a relatively modern (and Western) construction. Erotica from the turn of the 19th century tend to feature much curvier women than their modern equivalent. Women's preferences may be just as influenced by culture.

On the other hand, it is a rule of thumb in biology that females (of any species) are much pickier than males. There are good evolutionary reasons why that is so.

Even in humans, who share the burden of child-rearing more equally than many animals, having children requires far more of an investment of resources from a female than it does from a male. Women face strong incentives to spend their relatively limited reproductive resources on only the most attractive men, whose children will be most likely to breed in turn. Back to the sit-ups, boys.